The present invention relates to a system for controlled drug release within a vessel lumen, and to a method and to a device for controlled drug release.
A device for providing a continuous release of drugs over an extended period of time following from a single administration of a drug releasing material has wide application in treating disease. One type of continuous drug release mechanism is based upon degradation of biodegradable polymers. The biodegradable polymers have drugs incorporated within them. As the biodegradable polymers hydrolyze over time, the drugs are released. Hydroxycarboxylic acid polymers have been used to release drugs in this manner.
One other modality of drug release is a prolonged, though discontinuous release of drugs. Frequently, with a discontinuous release, there is a lag phase of no or negligible drug release when a drug delivery device is delivered to an in situ site for drug release.
One problem with sustaining drug release is that when drugs, particularly water soluble drugs, are incorporated into polymers, it is difficult to prevent a rapid, uncontrolled release of the drugs. As used herein, the term “water soluble drug” is defined as a hydrophilic compound with a solubility in water greater than 1 percent (w/v) and that is practically insoluble in nonpolar organic solvents such as ethyl acetate, methylene chloride, chloroform, toluene, or hydrocarbons. This rapid, uncontrolled release from a drug-polymeric matrix is known as a “burst effect.” The burst effect is particularly troublesome with high drug loading.
One other type of uncontrolled drug release is characterized by a “lag effect.” The lag effect occurs when the rate of drug release decreases to a negligible value.
The degree of drug release from a polymeric-drug matrix is, in part, controlled by the morphology of the polymeric-drug matrix. The morphology is, for some embodiments, a single-phase dispersion and for other embodiments, is a multi-phase dispersion. A single-phase dispersion is typically transparent when viewed in natural light. The single phase dispersion is clear and transparent because both the drug and the polymer have a mutual miscibility. A multi-phase dispersion has micro domains that give the dispersion a cloudy appearance. For some multi-phase dispersions, drugs are embedded in a polymeric matrix as particles.
Drug release is also controlled by the degree of drug loading. Matrices that have dispersed drug particles that do not contact each other tend to have a slow release of drug. A drug carrier such as blood is typically required to move the drug through the matrix and into the bloodstream of a living being.
Drug-polymeric matrices have been used to deliver drugs in situ through a vehicle such as a stent. The drug-polymeric matrix has been applied as a coating or a wrap to the stent. U.S. Pat. No. 5,605,696, which issued Feb. 25, 1997, describes a drug loaded polymeric material that is applied to an intravascular stent. The drug-polymeric matrix defines pores, multilayered to permit a combination of different drugs in a single stent. The stent also includes a rate controlling membrane that controlled retention and delivery of selected drugs to the affected blood vessel. The drug is dispersed as small particles, having a maximum cross-sectional dimension of 10 microns.